A MIMO (Multiple Input Multiple Output) system can increase the performance and communication capacity of a wireless communication system. MIMO employs multiple transmission antennas and multiple reception antennas to enhance data transmission and/or reception efficiency, and hence, is also called a multiple antenna system. MIMO techniques include spatial multiplexing, transmit diversity, beamforming, and the like.
In spatial multiplexing, independent symbol streams are transmitted in the same frequency bandwidth on different antennas of a transmitting node such as a base station (e.g., BTS, eNodeB, eNB, etc.) This allows data to be transmitted at high rates without increasing bandwidth of the system. In transmit diversity, the same data is transmitted from transmission antennas. By using space-time codes at the transmitting node, reliability of the detected symbols at a receiving node, e.g., a UE (user equipment) can be improved by exploiting transmit diversity. Beamforming is typically used to increase SINR (signal-to-interference-plus-noise ratio) of a signal by adding weight values according to channel states at multiple antennas. The weight values may be represented by a weight vector or a weight matrix, and is also referred to as a precoding vector or a precoding matrix.
In practical wireless systems such as the 3GPP (3rd Generation Partnership Project) LTE (Long Term evolution), UMTS (Universal Mobile Telecommunications System), HSDPA (High Speed Downlink Packet Access) and WiMAX (Worldwide Interoperability for Microwave Access) systems, knowledge of the channel or channels between the transmitting node and the receiving node is used to enhance performances. The channel knowledge can be made available at the transmitting node via feedback from the receiving node to the transmitting node. A MIMO transmitting node can utilize this channel information to improve the system performance with the aid of precoding. In addition to beam forming gain, the use of precoding avoids the problem of ill-conditioned channel matrix.
In wireless systems such as the ones mentioned above, multiple antennas with precoding and/or beamforming technology can be adopted to provide high data rates to the UEs. In these systems, the base station transmits one or more predetermined signals known in advance by UEs. These known signals are sometimes referred to as pilot signals (e.g., in UMTS) or as reference signals (e.g., in LTE). These pilot signals are inserted at predetermined positions in the OFDM (orthogonal frequency division multiplex) time-frequency grid and allow a UE to estimate the downlink channel so that it may carry out coherent channel demodulation. For ease of description, such known signals are referred to as pilot signals or more succinctly as pilots.
Another MIMO function for the pilots transmitted by the base station is for the UE to detect the pilots, and feed back to the base station an estimate of CSI (Channel State Information) based on the detected pilots. CSI refers to known channel properties of a communication link describing how a signal propagates from the transmitting node to the receiving node and represents the combined effect of, for example, scattering, fading, and power decay with distance. Based on the CSI estimate, the base station can adapt downlink transmissions to current channel conditions, which is important for reliable communication with high data rates in multi-antenna systems. Each MIMO channel between the base station and the UE needs its own CSI estimate.
In practice, complete CSI may be difficult to obtain, e.g., for a FDD (frequency division duplex) system. In such a system, some kind of CSI knowledge may be available at the transmitting node via the feedback from the receiving node. These systems are called limited feedback systems. There are many implementations of limited feedback systems such as codebook based feedback and quantized channel feedback. 3GPP LTE, HSDPA and WiMAX recommend codebook based feedback CSI for precoding.
In a codebook based precoding, predefined codebook is defined both at the transmitting and receiving nodes. Entries of the codebook can be constructed using different methods such as Grassmannian, Lyod algorithm, DFT matrix etc. The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix H (NR being the number of receive antennas at the receiving node and NT being the number of transmit antennas at the transmitting node), resulting in a so called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a signal subspace which is strong in the sense of conveying much of the transmitted energy to the UE. The signal subspace in this context is a subspace of a signal space that is defined in any number of dimensions including space, time, frequency, code, etc.)
In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced. At the receiving node, it is common to find SINR with different codebook entries and choose the rank and/or precoding index which gives the highest spectral efficiency (also referred to as channel capacity). In this context, rank indicates the number of data streams that can be simultaneously transmitted from a transmitting node to a receiving node.
The performance of a closed-sloop MIMO system generally improves with the cardinality (size) of the codebook set. At the receiving node, RI (rank information) and PCI (precoding control index) are sent back to the transmitting node every TTI (transmission time interval) or multiples of TTI (for example 5 in LTE, ⅓ in HSDPA).
Existing UMTS, LTE, and other systems (e.g., WiMax, 802.11(n), etc.) support up to 2×2 MIMO transmissions (max NR=2, max NT=2) which means that the base station must obtain two channel pilots to estimate or characterize each of the two spatial layers. In order to support 4×4 MIMO transmissions, the base station must obtain four channel pilots in order to estimate or characterize each of the four spatial layers. As compared to existing or legacy LTE systems, two new pilots must be defined to perform the channel demodulation and CSI estimation for the two new MIMO channels.
Pilots enable two main functionalities—CSI estimation through channel sounding where rank, CQI (channel quality information) and PCI are estimated and channel estimation for demodulation purposes. For a 4-branch MIMO (also referred to as 4Tx MIMO), the eNodeB may transmit four common pilots. In the context of this document, “common” pilots refer to pilot signals that are made available to all UEs and which are transmitted without UE specific beam forming.
Common pilots may be transmitted at instances in which legacy (e.g., 2×2 MIMO) UEs (Release 7 MIMO and Release 99) that are not able to demodulate the 4Tx transmissions, are scheduled. These legacy UEs cannot make use of the energy in the 3rd and 4th common pilots. Also the energy made available in the 3rd and 4th common pilots reduces the amount of energy available for HS-PDSCH (High Speed Physical Downlink Shared Channel) scheduling to the legacy UEs. Moreover, the 3rd and 4th common pilots can cause interference to these legacy UEs which at best can make use of the 1st and 2nd common pilots. Therefore, to minimize performance impacts to non-legacy (4Tx) UEs, it is desirable that the power of at least the 3rd and 4th common pilots be reduced to a low value. However, reducing the powers of the 3rd and 4th common pilots can negatively impact the performances of the non-legacy UEs.